In view of the prior art reference is made to publication U.S. Pat. No. 4,437,871 disclosing a bending and tempering method, where in the glass sheet is heated in a roller furnace in which the glass sheet is transported forward on rotating ceramic rollers. When the glass sheet is heated and reaches its softening temperature, its bending downwards between the rollers is prevented by the continuous movement of the glass sheet, wherein the location of supporting points, supporting the glass sheet from below, is continuously changed. In this manner, the glass sheet remains straight.
According to the method, heating of the glass sheet is continued until the glass sheet reaches also the tempering temperature which is approximately 620.degree. C. In the next step of the method, the glass sheet is supported by rollers in the furnace apparatus at a precisely predetermined point, a vacuum mould at said point above the rollers grips the glass sheet and elevates it upwardly apart from the rollers. This step of the method requires very high precision, because the positioning of the glass sheet must be precisely determined both in view of directional displacements and rotation. Positioning errors at this step result in faulty dimensions in the finished bend and tempered glass, particularly at its margins. An excessive position error will naturally damage the whole glass. The glass sheet is elevated by the vacuum mould up from the rollers to such an extent that a bending/tempering frame can be brought under the glass sheet from outside the furnace apparatus. The glass sheet is dropped onto the bending/tempering frame from a certain height, which is usually about 30 mm. After the dropping, the glass sheet is bent by gravity into the mould shape determined by the bending/tempering frame, and it is pulled out from the furnace apparatus. In the next step, the tempering is conducted with air jets. The bending of the glass sheet is relatively quick; with the most common glass thicknesses of 3-4 mm the bending will take about 1 to 3 seconds.
The method known from U.S. Pat. No. 4,437,871 is advantageous in certain respects, particularly in large-scale production for manufacturing simple bent and tempered glass shapes in large series. However, the method usually involves several disadvantages, the most important of which is the requirement that the glass sheet must be precisely positioned at the elevating step, which is difficult in quick serial production and causes losses, as mentioned above. Another disadvantage is the fact that the vacuum mould must be coated with a soft, heat-resisting coating material to prevent damage to the surface of the hot glass sheet. This coating material must be replaced from time to time, which will naturally cause production losses and replacement costs. Furthermore, application of the method to screencoated glass sheets will reduce the replacement time, because the silk screen ink used will stain the coating material, thus increasing the need for a replacement. Another disadvantage is also the fact that the glass sheet must, in practice, be dropped from a height of at least 20-25 mm, which may cause deformation of the glass sheet particularly at its margins.
Further in view of the prior art, a method is known from Finnish Patent No. 86054 where the glass sheet is first bent by gravity to a desired shape, using an edge mould. Thus the temperature of the glass sheet, particularly in its central areas, is clearly below 600.degree. C. After the bending stage, the glass sheet is supported by a pressure difference effective on its opposite surfaces, wherein the pressure is higher underneath than above the glass sheet, and the temperature of the glass sheet is raised thoroughly to the tempering temperature, i.e. about 620.degree. C. After this the glass sheet is quickly removed from the furnace apparatus to the tempering step. A quick move is necessary to prevent bending of the central areas of the glass sheet before the air jets of the tempering step cool and thus stiffen this area of the glass sheet below the softening temperature, ca. 550.degree. C. The method according to the Finnish Patent No. 86054 has been proved functional in practice, but the critical step in applying the method is the fact that the glass sheet is already bent to its final configuration before final heating. After the final heating and the elimination of a pressure difference supporting the glass sheet, it must be quickly moved to the tempering step. If in the abovementioned steps, for some reason, e.g. adjustments in the temperature or delays, excessive bendings or sags develop that exceed the manufacturing tolerance, these accumulated extra bendings may have the result that the bent and tempered glass deviates too much from the allowed manufacturing tolerances and must be discarded.